Final answer:
The rate of a reaction is determined by examining the change in concentration of reactants and products over time and is related to the stoichiometry of the reaction. The effect of changing reactant concentrations on the reaction rate can be predicted using the rate law and the rate constant.
Step-by-step explanation:
To determine the expression for the rate of a reaction, we examine the rates at which concentrations of reactants and products change over time. For a general reaction, the rate can be expressed in two ways: as the negative change in concentration of a reactant per unit time or as the positive change in concentration of a product per unit time. The stoichiometry of the reaction is used to relate these rates.
For example, if the rate at which reactant A decreases is given by -d[A]/dt, and A reacts to form product B at a stoichiometric ratio of 1:1, the rate of appearance of B would be expressed as d[B]/dt. These rates are equal once you account for the stoichiometric coefficients.
Collision frequency is key in understanding that as the concentration of a reactant increases, so does the rate of reaction. Conversely, a decrease in concentration leads to a slower rate of reaction. To complete the determination of the reaction rate, one would calculate the rate constant k with its appropriate units, which adjust depending on the overall order of the reaction expressed in the rate law.
The mathematical relationship in the rate law also allows us to assess the effect of concentration changes on the reaction rate. For instance, if the concentration of a reactant is doubled, the rate law can predict how this will affect the overall reaction rate, which could be a doubling, quadrupling, or another effect entirely based on the reaction order with respect to that reactant.